1. Field of the Invention
The present invention relates to a flat-shaped battery such as a button-type battery or a coin-type battery.
2. Description of Related Art
WO 02/013290 and JP 2003-151511 A disclose a flat-shaped battery including a dish-shaped positive electrode can (outer can) that is open upward, a dish-shaped negative electrode can (sealing can) that is open downward, and a gasket. In this flat-shaped battery, the edge of the positive electrode can and the edge of the negative electrode can are joined by caulking with the gasket interposed between them, thereby sealing the space between the positive electrode can and negative electrode can with the gasket. The flat-shaped battery disclosed in these documents uses the negative electrode can that includes a housing in the form of an inverted shallow dish for housing an electrode material (negative electrode material), and a first outer surrounding wall extending downward from the housing. Moreover, the lower end of the opening of the housing projects outward to form a flange, and a second outer surrounding wall is continuously formed at the outside edge of the flange. The second outer surrounding wall is folded back and joined together, so that double (inner and outer) walls are provided. The gasket is sandwiched between a surrounding wall of the positive electrode can and the second outer surrounding wall of the negative electrode can to seal the space between them.
The above flat-shaped battery is required to have a higher capacity. Therefore, it is necessary to increase the amount of an electrode material or electrolyte contained in the battery can of the flat-shaped battery. However, since the outside dimensions of the flat-shaped battery are predetermined, the battery capacity should be improved within the predetermined dimensions. One possible approach to this problem is to reduce the wall thickness of the battery can. To achieve an even higher capacity the outer surrounding wall of the negative electrode can may be formed of a single wall without folding, as disclosed in JP 7 (1995)-57706 A, JP 2003-68254 A, JP 4 (1992)-341756 A, and Japanese Patent No. 3399801.
However, the strength of the outer surrounding wall of the negative electrode can in the thickness direction is lower in the case of a single wall than in the case of a double wall. Therefore, when the surrounding wall of the positive electrode can is bent toward the axis of the positive electrode can by caulking, force is exerted on the outer surrounding wall of the negative electrode can in the thickness direction, and the outer surrounding wall can be easily bent toward the axis of the negative electrode can. This may result in a gap between the outer surrounding wall of the negative electrode can and the gasket, and thus cause a leakage of the electrolyte.